Conventionally a scissors jack for lifting a vehicle has comprised a leg, a first supporting arm attached to it pivotable around a horizontal pivot axis and having a load bearing member at its free end and a jack screw. One end of the jack screw engages a jack screw nut at the upper end of the leg and the other end engages in a jack screw bearing at a pivot joint connecting a second and third supporting arm or bracing arm with each other. The second supporting arm is pivotally connected to the first supporting arm. The third supporting arm is pivotally connected to the leg which can have a foot piece attached to its lower end.
The scissors jack of German Utility Model DE-GM No. 83 13 974.5 has a pivotally mounted foot piece. It obliges the operator of the scissors jack to adjust the leg into the properly slanted position to lift the vehicle, so that the load bearing member in all configurations of the scissors jack lies above the point of the foot piece and/or in the friction cone extending from the point of the foot piece to prevent the jack from slipping under a load.
Slipping of the scissors jack is a particular problem because a vehicle must be liftable from several different starting positions, namely from a lowest position in which the fully loaded vehicle has flattened tires and from other positions in which the vehicle is unloaded and has intact tires.
The friction cone is defined as a cone having as its apex the support point on the ground and an apex angle within which the horizontal force component of the load cannot overcome the friction resisting displacement so that no slip can occur if the load point lies within this imaginary cone. The cone angle is determined by the coefficient of sliding friction of the support point on the ground.
A scissors jack as described in German Open Application DE-OS No. 24 27 443 has a roller foot at the lower end of its leg provided with a plane contacting surface. With this roller foot and/or contacting surface the scissors jack and/or the leg is adjusted to the properly slanted position. However with this scissors jack it is necessary that it and its rolling foot be adjusted or fitted to the vehicle to be lifted. This scissors jack is satisfactory for many purposes but cannot be used universally for all types of vehicles.